In order to alter the electrical properties of semiconductor wafers, selected ions, such as arsenic, boron, or phosphorous ions, are used to dope the wafers. At first this doping was accomplished through the placement of trays of semiconductor wafers in diffusion furnaces. This arrangement, while at one time satisfactory, is not acceptable for today's increasingly smaller circuitry, as the deposition uniformities are not great enough.
Diffusion furnaces were replaced by ion implanters, which cause a beam of ions to be directed toward and impact upon a wafer in order to uniformly deposit ions on or in the surface of the wafer. Basically, these ion implanters include a source which generates a beam of ions to be implanted, which ion beam is passed through two perpendicular deflection systems, electrostatic or magnetic, to deflect the ion beam as required onto the wafer. A scan controller produces signals which control the charge placed on the deflection plates to provide a distribution of ions across the wafer much as an electron beam is selectively deflected to scan across the screen of a television set. Levels of non-uniformity of about one percent are presently possible with such a device.
This measure of non-uniformity is due, in part, to the prior lack of appreciation of the physical geometry of the ion beam striking the wafer. If the beam is exactly perpendicular to the center of the wafer, then as it progresses to the extremes of the wafer, the beam loses its perpendicular relationship. And as the angular speed of the beam is constant, the linear speed of the beam relative to the wafer as the beam is swept to the extremities increases This increase in speed causes fewer ions to be deposited at the extremities than at the center. Prior devices have attempted to solve this problem by placing a series of discrete step functions in the controller in order to slow the speed of the beam as it reaches the extremities of the wafer. This discreteness, by its very nature, leads to non-uniformities.
Another geometrical consideration which has not been fully appreciated is the fact that the wafer is generally tilted at an angle of about seven degrees from a plane which is perpendicular to the ion beam in order to prevent channeling of the ions through the wafer. Due to this tilted orientation the ions are trapped at the surface of the wafer. Quite naturally this tilted orientation means that there is another factor which will cause the beam to not travel at a uniform linear speed across the surface of the wafer.
Prior art devices additionally do not accurately control the beginning and ending of the ion beam scanning of a wafer. In fact, in order to come out to a one percent non-uniformity, the standard of the industry is to have approximately 100 scans of each wafer. Thus if the beginning of the first scan and/or the ending of the last scan lie partially through the wafer, the error falls within the one percent. Additionally beam walkover occurs when the ion beam is directed from an ion beam dump across the wafer to begin a scan at a position other than at the edge of the wafer. Ions are thus deposited in a non-uniform manner. Beam walkover can quite naturally occur when the ion beam is directed from a partially completed final scan back to the beam dump.
Further, prior commercial devices, due to improper selection of the frequencies which operate the two pairs of deflection systems, have a tendency to have scan lock wherein one frequency locks to the other, and thus there is striping of the doping on the wafer. These stripes are actually bands of heavier-than-average ion doping. These stripes of course affect the semiconducting properties of the chips which are eventually cut from the wafer. The ratio of the actually selected frequencies has been reported to be from 9 to 1, to 37 to 1. This selection causes small angled polygons to be formed in the ion pattern which causes excessive overscan of the wafer. Further this frequency selection, coupled with the fact that the ion beams are not stable in intensity, does not allow irregularities to be smoothed out well enough for the axis having the slower frequency.
Further, in order to more uniformly cover the wafer, prior devices have stepped the pattern described by the ion beam through a series of discrete positions in order to provide a more uniform application of ions to the wafer. This discrete approach by its very nature fails to eliminate non-uniformities.
As the above doping non-uniformities are not acceptable to many present day applications such as, for example, in high density gate array or memory chips, video cameras, microwave transistors, and A/D and D/A converters, the present invention is directed to overcoming the disadvantages of the prior art.